Harvesting damage caused by thinning of Norway spruce in unfrozen soil

Two harvester working methods and two types of forwarder equipment (tracks and chains) were compared with respect to harvester productivity, logging residue on strip roads, rut formation, and logging damage in the thinning of two Norway spruce stands in Southern Finland. In the normal harvester working method, trees removed further away from the strip roads were processed outside the strip roads. In the protective method, the harvester operator processed as many trees as possible on the strip road and turned some of the treetops parallel to the strip road. The protective harvester working method increased harvester time consumption by approximately 5%, but a higher proportion (86–92%) of the total logging residue potential was brought to the strip roads. The logging residue mass obtained to cover the strip roads was approximately 14–19 kg/m2. According to the recommendations for forestry practice in Finland, damaged trees should not exceed 4% of the number of the remaining trees, and ruts deeper than 10 cm should not exceed 4% of the total length of the strip road network. In our study, the proportion of damaged trees clearly exceeded the limit. After two to four forwarder passes, the proportion of deep ruts generally remained below that threshold. Neither logging damage nor rut depth differed between the harvester working methods. The prerequisites for a successful thinning operation performed on unfrozen soil are favorable weather, soil, and stand conditions. On dry soils with high bearing capacity, ruts remain shallow and the proportion of deep ruts is low. Furthermore, a high amount of logging residue decreases rut formation.